Optical information media include read-only optical disks such as compact disks, rewritable optical recording disks such as magneto-optical recording disks and phase change optical recording disks, and write-once optical recording disks using organic dyes as the recording material.
In general, optical information media can have an increased information density. Nowadays, optical information media are required to further increase their information density in order to process a vast quantity of information data as in images. The information density per unit area can be increased either by narrowing the track pitch or by reducing the interval between recorded marks or pits to increase a linear density. However, if the linear density is too high relative to the beam spot of reading light, the carrier-to-noise (C/N) ratio lowers, eventually to a level where signals are unreadable. The resolution upon signal readout is determined by the diameter of a beam spot. More illustratively, provided that the reading light has a wavelength λ and the optical system of the readout apparatus has a numerical aperture NA, the spatial frequency 2NA/λ generally becomes a resolution limit. Accordingly, reducing the wavelength of reading light and increasing the NA are effective means for improving the C/N and resolution upon readout. A number of technical studies that have been made thus far reveal that many technical problems must be solved before such effective means can be introduced.
Under the circumstances, several methods have been proposed for going over the resolution limit (or diffraction limit) determined by light diffraction. They are generally known as super-resolution readout methods.
The most common super-resolution readout method is to form a mask layer over a recording layer. Based on the fact that a laser beam defines a spot having an intensity distribution approximate to the Gaussian distribution, an optical aperture smaller than the beam spot is formed in the mask layer whereby the beam spot is reduced below the diffraction limit. This method is generally divided into a heat mode and a photon mode, depending on the optical aperture-forming mechanism.
The heat mode is such that upon irradiation to a beam spot, the mask layer changes its optical properties in a region whose temperature is raised above a certain value. The heat mode is utilized, for example, in the optical disk disclosed in JP-A 5-205314. This optical disk has on a transparent substrate in which optically readable recorded pits are formed in accordance with information signals, a layer of a material whose reflectance changes with temperature. That is, the material layer serves as a mask layer. The elements described in JP-A 5-205314 as the material of which the mask layer is constructed are lanthanoids, with Tb being used in Examples. In the optical disk of JP-A 5-205314, when reading light is irradiated, the reflectance of the material layer changes due to temperature distribution within the scanned spot of the reading light. After reading operation, the reflectance resumes the initial state as the temperature lowers. It never happens that the material layer be melted during reading. Another known example of the heat mode is a medium capable of super-resolution readout, as disclosed in Japanese Patent No. 2,844,824, the medium having a mask layer of an amorphous-crystalline phase transition material in which a high-temperature region created within a beam spot is transformed into crystal for increasing the reflectance. This medium, however, is impractical in that after reading, the mask layer must be transformed back to amorphous.
The heat mode media require that the power of reading light be strictly controlled in consideration of various conditions including the linear velocity of the medium since the size of the optical aperture depends solely on the temperature distribution in the mask layer. This, in turn, requires a complex control system and hence, an expensive medium drive. The heat mode media also suffer from the problem that reading characteristics degrade with the repetition of reading operation because the mask layer is prone to degradation by repetitive heating.
On the other hand, the photon mode is such that upon exposure to a beam spot, the mask layer changes its optical properties in a region whose photon quantity is increased above a certain value. The photon mode is utilized, for example, in the information recording medium of JP-A 8-96412, the optical recording medium of JP-A 11-86342, and the optical information recording medium of JP-A 10-340482. More illustratively, JP-A 8-96412 discloses a mask layer formed of phthalocyanine or a derivative thereof dispersed in a resin or inorganic dielectric, and a mask layer formed of a chalcogenide. JP-A 11-86342 uses as the mask layer a super-resolution readout film containing a semiconductor material having a forbidden band which upon exposure to reading light, is subject to electron excitation to the energy level of excitons to change light absorption characteristics. One illustrative mask layer is CdSe microparticulates dispersed in a SiO2 matrix. JP-A 10-340482 uses as the mask layer a glass layer in which the intensity distribution of transmitted light varies non-linearly with the intensity distribution of irradiated light.
Unlike the super-resolution readout media of the heat mode, the super-resolution readout media of the photon mode are relatively resistant to degradation by repetitive reading.
In the photon mode, the region whose optical characteristics change is determined by the number of incident photons which in turn, depends on the linear velocity of the medium relative to the beam spot. Also in the photon mode, the size of an optical aperture depends on the power of reading light, indicating that supply of an excessive power makes so large an optical aperture that super-resolution readout may become impossible. Therefore, the photon mode also requires to strictly control the power of reading light in accordance with the linear velocity and the size of pits or recorded marks (objects to be read out). Additionally, the photon mode requires to select the mask layer-forming material in accordance with the wavelength of reading light. That is, the photon mode media are rather incompatible with multi-wavelength reading.
Under the circumstances, JP-A 2001-250274 proposes a medium comprising a layer (functional layer) made of a specific material such as Si and having a specific thickness corresponding to the specific material. This medium enables to read out pits or recorded marks of a size which is below the resolution limit determined by light diffraction.
The medium of JP-A 2001-250274 is expected to find practical use because a carrier-to-noise ratio (CNR) of about 40 dB is available in reading out pits of a size which is below the resolution limit. Regrettably, the patent lacks the disclosure of an optimum method for enhancing read outputs.
It is reported in Jpn. J. Appl. Phys., Vol. 40 (2001), pp. 1624–1628 that in the readout operation on the medium of JP-A 2001-250274, some readout signals are omitted from a certain arrangement pattern of pits and spaces (which are regions between two adjacent pits). The pit train of alternately arranged pits and spaces usually includes pits and spaces of differing lengths depending on a particular modulation system used, and these pits and spaces are arranged in a pattern compliant with the modulation system and the information to be recorded. If the arrangement pattern which causes some readout signals to be omitted is identified, then the modulation system can be devised such that the signal-omitting arrangement pattern may not develop. However, the resulting modulation system has increased redundancy and is detrimental to the efforts of increasing the capacity of media. All the patent and literature references cited above are incorporated herein by reference.